The present invention relates to a buffer amplifier that generates an output voltage in accordance with an input voltage, and more particularly, to an amplifier circuit having a wide output voltage range.
In the prior art, a buffer amplifier that outputs an output signal amplified at a gain of one time is used in accordance with the provided input signal. When an operational amplifier is used as the buffer amplifier, feedback is necessary. Thus, the circuit becomes more complicated. To solve this problem, the use of a driver circuit having low power consumption and high voltage accuracy has been discussed (refer, for example, to Japanese Laid-Open Patent Publication No. 2005-217949, FIG. 3). Such a driver circuit includes a voltage generation unit for generating a basic output voltage that is in accordance with an input voltage, a first buffer circuit for outputting an output voltage in accordance with the basic output voltage output from the voltage generation unit, and a second buffer circuit for generating a voltage in accordance with the output voltage. The second buffer circuit consumes more power than the first buffer circuit. The driver circuit further includes a quasi-buffer circuit having substantially the same properties as the first buffer circuit. The quasi-buffer circuit generates a quasi-voltage in accordance with the basic output voltage provided by the voltage generation unit. The basic output voltage is controlled based on the simulation voltage.
There has also been discussion of a buffer amplifier having an output voltage range that can be widened (refer, for example, to Japanese Laid-Open Patent Publication No. 2002-185269, FIG. 1). Such a buffer amplifier includes an n-channel source follower circuit and a p-channel source follower circuit, each of which receives an input voltage. The buffer amplifier further includes an output stage circuit connected to the outputs of the two source follower circuits to generate an output voltage. A first current mirror circuit supplies an output current to the output of the output stage circuit using the output current of the n-channel source follower circuit as an input current. A second current mirror circuit supplies an output current to the output of the output stage circuit using the output current of the p-channel source follower circuit as an input current.
However, the output voltage range (swing) is not sufficient for the following reasons. Referring to FIG. 14, a schematic diagram of the driver circuit disclosed in Japanese Laid-Open Patent Publication No. 2005-217949 is shown. In this driver circuit, an output of a pulse generator 500 is amplified by a first buffer circuit 510 and a second buffer circuit 520. In this circuit, however, the output voltage of the second buffer circuit 520 is restricted by the base-emitter voltage of a transistor 531 when the input voltage is in a high range and restricted by a transistor 532 when the input voltage is low.
FIG. 15 is a schematic circuit diagram of a buffer amplifier of Japanese Laid-Open Patent Publication No. 2002-185269. The buffer amplifier includes current mirror circuits 640 and 650 to widen or increase the swing. The current mirror circuits 640 and 650, respectively, include transistors 641 and 642 and transistors 651 and 652 to generate output voltages in a range in which source follower circuits 610 and 620 do not function.
In order to operate the transistor 641, however, the drain voltage of the transistor 611 must be decreased as much as possible. When the input voltage becomes high and approaches the power supply voltage, the drain voltage of the transistor 611 also becomes high. Thus, the transistor 641 cannot be sufficiently driven.
In this case, the drive capacity of the current mirror circuit 640 can be ensured by increasing the threshold voltage of the transistor 611. However, this would lower the operational speed since the source-drain voltage of the transistor 611 is small. Such a problem also arises with the source follower circuit 620 and the current mirror circuit 650 when the input voltage is low.